1. Field of the Invention
The present invention relates to a turbocharger having a casing system which houses a centrifugal impeller connected to a turbine by a shaft.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Industrial turbochargers, particularly for marine application, are made so that, if the compressor impeller were to burst, the surrounding casings would be capable of containing all the impeller fragments. Marine certification societies dictate that impeller hub burst containment must be demonstrated at turbocharger rotational speeds 20% in excess of the maximum allowable operational speed.
When an impeller bursts, there are two main mechanisms whereby fragments might not be contained by the casings. The first is penetration of the casings by impeller fragments. The second results from failure of fixings holding casings together, allowing gaps to appear between casings, and impeller fragments to escape through the gaps.
The impeller can be designed so that fragments originating from its outermost portion are of low mass (and therefore low energy). Typically, therefore, a state of the art impeller is designed with a relatively thin hub region over its outer portion.
FIG. 1 shows schematically a sectioned view through a turbocharger impeller and a casing system housing the impeller. The impeller has a hub 1 with an outer annular rim 2. The hub also has a front face 3 and a rear face 4 which converge towards the rim from respectively the inlet side and the shaft side of the hub. An annular balance land 5 projects from the rear face 4 of the hub. The casing system includes a volute casing 6 which forms a volute for receiving compressed air from the impeller, and a separate insert casing 7 which inserts from the inlet side of the impeller into the volute casing to form a duct for feeding air to and through the impeller. The casing system also includes a main casing 8 which forms a housing for the shaft and for the shaft-side end of the impeller.
A seal plate 9 attached to the main casing 8 forms a labyrinth seal 10 with the rear face 4 of the hub 1 outward of the balance land 5 and close to the outer face of the rim 2. The seal plate also extends radially outwardly to carry the rear wall of an annular passage 11, optionally containing diffuser vanes 12, which directs compressed air from the impeller to the volute. The balancing land 5 produces a neck region 13 of reduced thickness immediately inboard thereof.
During a hub burst, cracks generally initiate in the hub 1 near the impeller centre-line. As they propagate outwards, cracks also form in the neck region 13, allowing the impeller rim 2 to be shed. Fragments of the rim pass through any diffuser vanes 12 (which offer little resistance due to their relatively flimsy structure) and then impact on the wall of the volute casing 6. This wall is therefore usually thickened to prevent penetration. The remaining larger pieces of the hub 1, of relatively small outer diameter once the rim 2 has been shed, impact the insert casing 7 immediately surrounding the impeller. Typically this casing shatters, absorbing the energy of the hub fragments.
Increasingly, higher pressure ratios are being demanded from industrial turbochargers. As a result, rotational speeds of impellers are increasing and impeller designs must be altered to allow for consequently increasing stresses. Typically therefore the shape of the impeller hub is made more wedge-shaped (i.e. the angle between its front and rear faces is increased) to support the added centrifugal loads from the impeller rim. This in turn means that the neck region in the impeller is displaced to a higher diameter, and the rim, outboard of the neck, is reduced in size. Impeller designs suitable for higher pressure ratios tend to have a narrower operating range: the usable range of mass flow at a particular pressure ratio is low compared to impellers designed for lower pressure ratios. To overcome this tendency, the wall of the duct which feeds air to and through impeller may incorporate slot-shaped apertures.
Since the rim of the impeller is smaller (albeit rotating at higher speed at failure), the energy that must be dissipated to prevent penetration of rim fragments does not necessarily increase in line with the rotor speed. As a result, only a modest increase in the thickness of the volute casing may be necessary to prevent penetration by such fragments.
However, the remaining hub fragments are larger, extend to a higher radius and are more wedge-shaped. In combination with the higher speed at failure, this results in the remaining hub fragments containing substantially greater amounts of energy. As a consequence, although the shattering of the insert casing absorbs some of their energy, the wedge fragments may not be blunted and may pass intact, still with significant energy, beyond the insert casing, thereby potentially avoiding containment.